1. Field
This document relates to a touch sensor driving device and a display device comprising the same.
2. Related Art
User interfaces (UI) enable humans (users) to interact with various types of electric or electronic devices so that they can easily control the devices as they want. Typical examples of the user interfaces include keypads, keyboards, mice, on-screen displays (OSD), and remote controllers with an infrared communication capability or radio frequency (RF) communication capability. User interface technology is continuing to make progress toward higher user sensitivity and ease of operation. Recently, user interfaces have been evolving into touch UI, voice recognition UI, 3D UI, etc.
The touch UI has been iadopted in portable information appliances. The touch UI is implemented through a method for forming a touchscreen on the screen of a display device. Such a touchscreen can be implemented as a capacitive touchscreen. A touchscreen having capacitive touch sensors detects touch input by sensing a capacitance change, i.e., a change in the amount of charge in the touch sensors when a finger or conductive material comes into contact with the touch sensors.
The capacitive touch sensors can be implemented as self-capacitance sensors or mutual capacitance sensors. The electrodes of the self-capacitance sensors are connected to sensor lines oriented in one direction on a one-to-one basis. The mutual capacitance sensors are formed at the crossings of orthogonal sensor lines Tx and Rx with a dielectric layer interposed between them.
A touchscreen having capacitive sensors is connected to a plurality of touch sensing circuits. Each touch sensing circuit senses a change in the amount of charge in the touch sensors by receiving a touch sensor sensing signal from the touchscreen through a receiving channel. These touch sensing circuits may be integrated in a touch sensor driving device (integrated circuit) and connected to the sensor lines of the touchscreen.
An example of the touch sensing circuits is depicted in FIGS. 1 and 2. FIG. 1 illustrates a touch sensing circuit when a touchscreen TSP is implemented using mutual capacitance sensors Cm. FIG. 2 illustrates a touch sensing circuit when a touchscreen TSP is implemented using self-capacitance sensors Cs.
The touch sensing circuit of FIG. 1 may comprise an OP amp OP and a sensing capacitor Cf. An inverting input terminal (−) of the OP amp OP may be connected to a touch sensor Cm through a receiving channel, a non-inverting input terminal (+) of the OP amp OP may be connected to an input terminal of reference voltage Vref, and an output terminal of the OP amp OP may be connected to the inverting input terminal (−) via the sensing capacitor Cf.
In the touch sensing circuit of FIG. 1, the OP amp OP operates as an inverting amplifier. The touch sensing circuit's output voltage Vout may be represented as in Equation 1.Vout=Vref−Vtx*(CM/CF)  [Equation 1]
where the reference voltage Vref is a DC level voltage, Vtx represents a touch driving voltage applied to the mutual capacitance sensor Cm, CM represents the mutual capacitance of the mutual capacitance sensor, and CF represents the capacitance of the sensing capacitor Cf. The output voltage Vout of FIG. 1 indicating a change in the amount of charge in the mutual capacitance sensors Cm has the opposite phase to that of the touch driving voltage Vtx.
The touch sensing circuit of FIG. 2 also may comprise an OP amp OP and a sensing capacitor Cf. An inverting input terminal (−) of the OP amp OP may be connected to a touch sensor Cs through a receiving channel, a non-inverting input terminal (+) of the OP amp OP may be connected to an input terminal of touch driving voltage Vm, and an output terminal of the OP amp OP may be connected to the inverting input terminal (−) via the sensing capacitor Cf.
In the touch sensing circuit of FIG. 2, the OP amp OP operates as a non-inverting amplifier. The touch sensing circuit's output voltage Vout may be represented as in Equation 2.Vout=Vm+ΔVm*[1+(CS/CF)]  [Equation 2]
where Vm represents a touch driving voltage applied to the self-capacitance sensor Cs, ΔVm represents the amplitude of the touch driving voltage Vm, CS represents the self-capacitance of the self-capacitance sensor Cs, and CF represents the capacitance of the sensing capacitor Cf. The output voltage Vout of FIG. 2 indicating a change in the amount of charge in the self-capacitance sensors Cs has the same phase as the touch driving voltage Vm.
The permissible range of the output voltage Vout of a touch sensing circuit is determined in advance at the design stage, in consideration of the size of the touch sensor driving device. As display devices are getting larger, the size of the touchscreen TSP is growing and the mutual capacitance or self-capacitance of the touch sensors is also increasing. An increase in the capacitance CM or CS of the touch sensors leads to an increase in the absolute value of the output voltage Vout of the touch sensing circuit, as in Equations 1 and 2. In this case, the output voltage Vout of the touch sensing circuit may exceed a given permissible range and become saturated. Since the presence or absence of a touch is detected depending on how high the output voltage Vout of the touch sensing circuit is, it is impossible to tell whether there is a touch or not if the output voltage Vout exceeds the permissible range and becomes saturated.